Sound translating device and method of making the same



June 25, 1940. H. c. PYE 2,205,669

SOUND TRANSLATING DEVICE AND METHOD OF MAKING THE SAME Filed July 22, 1938 INVENTOR.

Harold E. F'L E BY MM ATTORNEYS.

Patented June 25, 1940 UNITED STATES SOUND TRANSLATING DEVICE AND METHOD OF MAKING THE SAIWE Harold C. Pye, Oak Park, Ill., assignor to Associated Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application July 22, 1938, Serial No. 220,758

15 Claims.

The present invention relates to sound translating devices and methods of making the same and more particularly to improvements in telephone receivers of the permanent magnet type 5 and methods of making the same.

In a sound translating device of the type mentioned, a magnetic field structure is provided which conventionally comprises a pair of pole pieces upon which the voice coils are wound and having pole faces disposed adjacent the diaphragm of the device. The magnetic field structure also includes a permanent magnet, usually of the bar type, arranged between the pole pieces to produce a'steady magnetic field which links the diaphragm. In the past, it has been the practice to magnetize the bar magnet before it is assembled in the sound translating device and to enclose the magnetic field structure assembly in a housing. The prior method of magnetizing ml the magnet bar before assembly is open to the criticism that iron or steel filings or other minute magnetic particles may be picked up by the field structure during the assembly operations unless great care is exercised. Such magnetic particles Q5 may become embedded in the turns of the voice coils ultimately to cause short circuits or may become lodged between the pole faces and the diaphragm to interfere with the proper response of the diaphragm to signal currents traversing the voice coils. Moreover, when the entire field structure'is enclosed within a housing, the only practical method of constructing the device is to magnetize the magnet bar before the assembly operations are started.

3 It is an object of the present invention to pro-' vide an improved sound translating device of the character described which is of extremely simple, economical and rugged construction, is compact in arrangement, and is efficient in operation.

It is another object of the invention to provide a" sound translating device of the character described which includes a housing for the windings and diaphragm and wherein the parts of the mag-' netic field. structure are so arranged that the magnet bar may readily be magnetized after the device is fully assembled.

It is a further object of the invention to provide, in a sound translating device which includes a non-magnetic conductive housing electrically coupled with the magnetic circuit of the field structure, an improved field structure which is so arranged that alternating signal currents traversing the field windings are not substantially dissipated as hysteresis and eddy current losses in the housing. 7

It is a still further object of the invention to provide an improved method of making a sound translating device of the character described, the steps of which are so arranged that the accumulation of foreign magnetic particles upon the field *5 structure is substantially prevented.

In general, the objects as set forth above are in part attained in accordance with the present invention by providing a sound translating device which comprises a substantially non-magnetic 10 conductive frame or housing and a magnetic field structure mounted on the housing so that the magnetic circuit of the structure is'coupled to the housing, together with means comprising an auxiliary flux path extending between the pole 1 5 pieces of the field structure for preventing at least a portion of the alternating component of flux which is developed during operation of the device from linking with the frame or housing.

In the arrangement illustrated, the auxiliary flux 20 path comprises projections extending from the pole pieces toward each other and separated by an air gap. More particularly, the housing or frame is provided with a pair of openings extending therethrough and the pole pieces are provided with extensions projecting through the openings to have the permanent magnet of the field structure secured therebetween on the outside of the housing.

In accordance with a further feature of the 30 invention, the device is constructed in accordance with an improved method which comprises the steps of fully assembling the parts of the device before the magnet bar is magnetized and thereafter magnetizing the bar. More particularly, the 5 method of making the device comprises the steps of inserting the pole piece extensions, mentioned above, through the openings provided in the housing so that they project outside the housing; positioning the magnet member between the projected 1 portions of the pole piece extensions; securing the magnet member between the pole piece extensions and the pole pieces to the housing; mounting the diaphragm structure on the housing to provide an enclosed chamber within which the pole pieces 4 and the windings of the field structure are positioned; and thereafter magnetizing the magnet member.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and the method of making the same, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection Cally illustrates the method of magnetizing the.

is an elevational View or" the device shown in Fig. l l

with the diaphragm assembly removed therefrom,

Fig. 3 is an exploded view illustrating the method of ass'embling'the device shown in'Fig; 1, Figs. 4 and 5 are detail views of certain'of the elements shown in Figs. 1 and 3, and Fig. 6 diagrammatipermanent magnet member embodied in the field structure of the device shown in Fig. 1.

Referring now more particularly to Figs. 1, 2

and 3. of the drawing, there is illustrated a sound translating device in the form of a telephone receiver which comprises a cup-shaped housing Hi constructed of non-magnetic conductive material, such, for example, as aluminum, andhaving mounted thereon the magnetic field struc ture of the device. Therim ofthe housing 10 is provided witha bent-over flanged portion; H having formed integrally therewith an annular ring I2, theoutersurface of which is ground smooth to form a bearing surface for a receiver diaphragmifi. Pref-erablythe diaphragm 13 has a thickness of approximately .008 inch and is constructed of magnetic material, such, forexample, as silicon steel, having a high A. C. permeability. The magnetic field structure of the device comprises a pair of polepieces M and I5, upon which the voice coils l6 and I! of the receiver are wound,-and a permanent magnet 18 of the bar type which, in conjunction with the diaphragm l3 and the pole pieces l4 and I5, forms a substantially closed magnetic circuitinterrupted only by the narrow air gaps extending betweenthe pole faces of the pieceslfi and i5 and the diaphragm l3. More particularly, the pole pieces l4 and i5 are respectivelyprovi ded with offset extensions 14a and '59. which extend through snugly engaging spaced apart, openings I 9 and 20 cut in the bottom of'the cup-shaped housing i0, andv are arranged to receive the magnet member 18 therebetween. For the purpose of "retaining the magnet bar is within the pole piece extensions 4a and [5a, the upper corners, and more particularly the edges, of the magnet l8 are beveled as indicated at 189. and Hip, and

the upstanding corner portions of the pole piece extensions Maand 5a are bent over to engage the beveled edges of the magnet bar in the manner best illustrated in Fig. 5. Preferably, the length of thebar is just slightly greater than the spacing between the pole piece extensions [4a and 5a so that the oppositely .disposed faces of v the pole piece extensions Ma and 5a; firmly engage the adjacent faces of the magnet bar. to provide a low reluctance connection between the pole pieces and the magnet bar. It will be noted that'the mounting arrangement just. described serves to restrain .the magnet bar I8 against movement in any direction and, thus, this bar is rigidly held in, place when the assembly of the device is completed. i

The magnet bar 18 ispreferably formed of an alloy consisting of twenty-four to twenty-six per cent nickel, eleven to thirteen per cent aluminum and sixty-one to sixty-five per cent iron, such for example as the alloyknown as Alnic; While the pole pieces 14 and 15 are formed of an alloy comprising approximately 45 per cent nickel and the, housing.

the windings It are brought out through slots last-mentioned winding are brought out through slots providedin the inner side walls of the winding heads 24 and 25 to anchor studs 2% and 25a. formed integrally with and projecting from the winding heads 24 and 25, respectively. In accordance with conventional practice, the two windings i6 and l! are connected inseries by soldering together the ends of the windings terminating at the anchor studs 22a and 24s. The remaining two winding terminals are brought out and connected to the connecting terminals of the device which, in the arrangement illustrated, comprises the conductive housing 10' and a U-shaped contact member 2! having its legs straddling the magnet member l8 and secured tothe housing It]. I More specifically, the ends of the legs of the contact member 27 are bent over and provided with holes 28s. fi1'ld'28b therein which register with holes 292. and 291, formed in the housing In. The contact member 21 is mounted upon thehousing H! by rivets 30a and 30b. extending through the openings 28g, 299. and 28b, 295, respectively, and is insulated from the housing lll'by insulating washers 3'21, 3h), 32:; and 32b, in the manner illustrated.

As indicated above, the housing I0 through. which the pole piece BXtEHSiOHS IQa and |5a extend is closely coupled with the magnetic circuit of the field structure and effectively constitutes a short-circuited conductor encircling each of the two pole pieces I4 and I5. The close inductive couplings between the housing and the magnetic circuit of the field structure results from the fact that the side walls of the openings I9 and 20 closely embrace. the pole piece extensions Ma and l 59, to provide support for the field structure and to prevent the entrance of'dirt and moisture within the housing It. With this arrangement,'alternating or undulating flux traversing the pole pieces M and I5 and the magnet bar is necessarily caused induced voltages to be set up in the conductive housing I0 so that a portion of the alternating current energy producing the alternating flux is necessarily dissipated as hysteresis and eddy current losses in In order to prevent alternating flux developed during operation of the device from .linking with the housing I0, there is provided Within the housing an auxiliary flux path extending between the two polepieces. This auxiliary flux path comprises projections Mn and I51; formed integrally with the pole pieces I4 and I5, respectively, and extending toward each other. The adjacent ends of the'projections Mn and Hit are separated by an air gap 33 which is so proportioned that the magnet bar I8 is not effectively short-circuited and a substantial amount of the steady state flux produced by the magnet I8 is caused to traverse the diaphragm l3. Preferably, this air gap is of the order of .0625 inch. The pole pieces M and I5 are preferably stamped from flat stock and during the stamping operation there are cut in the projecting portions I4}, and |5b thereof a plurality of openings 34 and 35, which are utilized in securing the pole pieces to the housing In. More particularly, the pole pieces l4 and I5 are mounted upon the housing Ill by means of rivets or bolts 36 and 31 extending through the openings 34 and 35, respectively, and through openings 38 and 39 formed in the bottom portion of the housing l0 and spaced to register with the openings 34 and 35.

The diaphragm assembly comprises, in addition to the diaphragm l3, an inner plate 40, an ear piece 4|, an annular supporting ring 42, and an assembly ring 43. As disclosed and claimed in co-pending application Serial No. 220,759, filed July 22, 1938, Harold C. Pye, the inner plate 40, in conjunction with the diaphragm l3 and the annular ring l2, defines a damping chamber adjacent the inner surface of the diaphragm 13, while the ear piece 4|, in conjunction with the diaphragm l3, defines a resonating chamber adjacent the outer surface of the diaphragm. The dimensions of the damping chamber and of the restricted passages communicating therewith are so proportioned that the peak in the response characteristic of the device caused by the natural period of vibration of the diaphragm I3 is substantially reduced to flatten the over-all fre quency-response characteristic of the device. More particularly, the inner plate 40 which is constructed of non-magnetic material, such, for example, as aluminum, is provided with six openings 44 and has mounted thereon in spaced apart relation a fiat annular ring 45 which defines a relatively long, narrow, restricted and annular passage 46 communicating between the interior of the housing In and the chamber formed between the plate 40 and the diaphragm l3. By suitably proportioning the size of the holes 44, and the length and width of the restricted passage 46, the acoustical impedance of the chamber adjacent the inner surface of the diaphragm |3 may be properly proportioned to achieve the desired maximum damping effect over the band of frequencies within which the natural period of vibration of the diaphragm I3 is most effective to cause distortion of the over-all response characteristic. Also the layer of air in this chamber serves artificially to enhance the stiffness of the diaphragm l3, thereby to improve the fidelity of reproduction of the device. Although given by way of example only, the following specifications for the parts defining the dimensions of the damping chamber and the passages communicating therewith have been found to be highly satisfactory in practice:

Inches Diameter of chamber 1.683 Depth of chamber 0.030 Diameter of holes 44 0.076 Depth of passage 48 00030-00040 Width of ring 45 .319 Outer diameter of ring 44 1.444

As best shown in Fig. 4, the annular ring 45 is separated from the inner plate 40 by means of thin washers 41 and is mounted upon the plate 40 by means of rivets 48 extending through the washers and registering openings 49 and 50 respectively provided in the plate 40 and the ring 45. As best shown in Fig. 3, the openings 49 and 50, through which the mounting rivets 48 extend, are staggered so that the assembly comprising the plate 40 and the ring 45 is prevented from buckling. The plate 40 has formed therein a pair of slots 40a and 40b through which the pole face ends of the pole pieces l4 and I5 extend. A cross slot 400 connecting the two slots 40a and 40b is also provided for preventing the circulation of eddy currents in the plate 40. The dimensions of the slots 40a and 40b are substantially greater than the cross-sectional dimensions of the pole face ends of the pole pieces M and I5 '50 that relatively large air gaps are provided between the adjacent surfaces of the pole pieces. and the plate 40. By virtue of this arrangement, the coupling between the magnetic circuit of the magnetic field structure and the plate 40 is extremely loose and the production of eddy currents in the plate is substantially prevented.

The ear piece 4|, which, in conjunction with the diaphragm l3, defines a resonating chamber adjacent the outer surface of the diaphragm, is provided with a centrally disposed opening 5| through which sound waves are transmitted from the resonating chamber to the exterior of the dev1ce. projecting annular portion 52 having a smooth upper surface which is adapted to cooperate with the oppositely disposed lower surface of the annular ring portion l2 of the housing I!) to clamp the peripheral edge of the diaphragm l3 there-- between. The clamping surfaces of the two annular portions l2 and 52 are so formed that the peripheral edge of the diaphragm |3 is clamped therebetween with uniform pressure at all points around the outer circumference of the diaphragm. Preferably, the dimensions of the outer resonating chamber and of the passage 5| are so proportioned that the natural period of vibration of the chamber occurs at approximately 2,700 cycles per second, whereby the response of the moving.

system is materially enhanced at frequencies lying within the band extending from 2,400 to 2,800

cycles per second. To achieve this greater response in the frequency band indicated, it has been found that the following dimensions for the 2 resonating chamber and the passage 5| are satisfactory:

Inches Outer diameter of chamber adjacent diaphragm 1.688 Maximum depth of chamber 0.033 Diameter of passage 5| 0.250 Depth of passage 5| 0.051

In constructing the device described above,

the housing 0 in the manner explained previously, and the extensions Ma and His of the pole pieces l4 and I5 are inserted through. the openings l9 and 20, respectively. The last-mentioned operation is performed with the windings IE and I1 fully assembled on the pole pieces I4 and I5. After the pole piece extensions Ma and |5a have been inserted through the openings l9 and 20, the magnet member l8, which is at this time demagnetized, is forced between the adjacent op" posite side surfaces of the pole piece extensions, and the extensions are pressed toward each other to bring the inner surfaces thereof into firm engagement with the pole faces of the magnet member l8. Simultaneously, the projections Mb and I 5b of the pole pieces [4 and I5 are pressed against the bottom Wall of the housing I 0. While the above-mentioned forces are acting upon the pole pieces 14 and 15, the rivets 36 and 3'! are inserted through the openings provided in the housing l0 and the pole pieces, and the ends thereof are flattened in the usual manner, thereby rigidly to mount the field structure upon the housing I ll. Also, while the above-mentioned forces are being exerted upon the pole pieces Hi and E5, the upstanding corner portions of the pole piece extensions Ma and [5a. are bent over against the beveled corners I89. and 18b of the magnet member 18. After the field structure assembly is completed in the manner just described, the diaphragm assembly is mounted upon the rim of the cup-shaped housing lit. To this end, the inner plate 40 is forced within the snugly engaging side walls of the annular ring I2 to abut against the ledge formed by'the lower surface of the housing flange II. This operation is performed with the fiat annular ring mounted upon the plate 40, and during the performance thereof the pole face ends of the pole pieces'lfl and I5 pass through the slots Mia and 48s pro.- vided in the plate lll. The inner plate 4!! is rigidly held within the annular portion [2 of the housing Ill by staking the inner surface of the ring portion 12 at spaced points around the circumference thereof in a well-known manner. Also, a sealed joint is provided between the plate 40 and the pole face ends of tr e pole pieces i l and I5 by pouring an insulating compound, such, for example, as Bakelite cement, through the spaces between the sidewalls of the slots 49a and 40b and the side surfaces of the pole pieces I 3 and I5, thereby to insure that the damping chamber shall only be connected with the interior of the housing It by way of the restricted passage 46 described above. The next step in the construction of the device is to clamp the diaphragm l3 between the clamping surfaces of the two ring portions l2 and 52 projecting respectively from the housing l0 and the ear piece 4|.

'While the diaphragm is clamped in position, the

retaining ring 43 with the annular ring 42 inserted therein is telescoped over the ear piece 4| until the flanged portion 439, thereof engages the tapered surface of the ear piece 4|, following which the rim of the retaining ring 43 is bent over as indicated at 43b to engage the underside of the flanged portion H of the housing it.

The final step in the construction of the device is that of magnetizing the magnet member l8. To this end, and as best shown in Fig. 6, the assembled device is positioned so that the magnet member l8 lies between the poles 53a and 53b of an electromagnet 53 having its winding 54 connected through the contacts of a circuit breaker 55 to a direct current source, not shown, but having its positive and negative terminals connected to the terminals 56. In order to expedite the magnetizing operation, the poles 53a and 53b of the magnet 53 are provided with cut-away portions 51a and 57b within which the legs of the U-shaped contact member 27 and the heads of the rivets 3th and 301; are adapted to extend. By

this arrangement, the pole piece extensions [4a and '59. may be brought into close proximity with the pole faces of the poles 53b and 53a, respectively, so that a minimum flux leakage occurs. When the magnet member I8 is fully magnetized, the last step in the construction of the device is completed and the device is ready for use.

In the operation of the device constructed and arranged in the manner set forth above, anattractive force is normally exerted upon the diaphragm I3 by virtue of the flux produced in the magnetic field structure by the magnet member l8. Due to the length of the air gap 33, the magnet member is not short-circuited and a substantial portion of the fiux traverses the diaphragm l3. When incoming signal currents traverse the voice coil windings I6 and H, the steady state flux is increased and decreased in the usual manner in accordance with theundula- ,tions of the signal currents. The change in the flux traversing the pole pieces l4 and I5 causes induced voltages to be developed in the closely coupled conductive housing l0 and the result-- ing circulating currents cause back electromoti-ve forces to be developed which oppose the flux changes in the usual manner. Thus, the reluctance of the portion of the magnetic circuit com-- prising the permanent magnet l8 and the-pole piece extensions Ida and [5a is effectively increased so that the greater portion of the alternating component of the flux traverses theauxiliary path described above and comprising the pole piece projections 14b and [5b and does not link withthe housing 10. As a result, only a small portion of the signal current energy is dissipated as hysteresis and eddy current losses in the housing In. The variations in the vflux traversing the diaphragm l3 causes this diaphragm to vibratein the usual manner so that sound waves are transmitted by way of, the opening 5| to the exterior of the ear piece 4| and the provision of the inner damping chamber defined by the plate 46 prevents an excessive response of the diaphragm 13 at frequencies appreaching the natural frequency of vibration of the diaphragm. Also, the provision of the resonating chamber defined by the ear piece 4| enhances the response of the moving system at frequencies in the upper portion of the operating range. Thus, a substantially flat response characteristic typifying good fidelity of reproduction is obtained.

While there has beendescribed what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such .modifications as fall within the true spirit and scope of they invention.

I claim: I

A sound translating device comprising a substantially non-magnetic and electrically conductive frame, a magnetic field structure mounted on said frame, said frame being inductively coupled to the magnetic circuit of said structure, and means comprising said frame for preventing at least a portion of the alternating component of flux which is developed during operation of said device from linking with said frame.

2. A sound translating device comprising a non-magnetic and electrically conductive frame, a magnetic field structure mounted upon said frame and comprising a magneticcircuit including two paths, one of said paths including a permanent magnet, said frame being inductively coupled to the magnetic circuit of said structure, and means comprising said frame for causing the predominant portion of the alternating component of flux which is developed during operation of said device to traverse the other of said paths. I 1 a 3. A sound translating device comprising a subtive frame, a diaphragm, a magnetic field structure mounted on said frame and comprising a pair of pole pieces arranged to cooperate with said diaphragm to provide a substantially closed magnetic circuit, said frame being arranged to encircle at least one of said pole pieces, thereby to affect a short-circuited conductor inductively coupled to said magnetic circuit, and means comprising said frame and a relatively low-reluctance auxiliary flux path extending between said pole pieces for preventing at least a portion of the alternating component of flux which is developed during operation of said device from linking with said frame.

4. A sound translating device comprising a sub stantially non-magnetic and electrically conductive frame, a diaphragm, a magnetic field structure mounted on said frame and comprising a pair of pole pieces arranged to cooperate with said diaphragm to provide a substantially closed magnetic circuit, said frame being arranged to encircle at least one of said pole pieces, thereby to affect a short-circuited conductor inductively coupled to said magnetic circuit, said pole pieces having projections extending toward each other and separated by an air gap to provide an auxiliary flux path between said pole pieces, and means comprising said frame and said auin'liary flux path for preventing at least a portion of the alternating component of flux which is developed during operation of said device from linking with said frame.

5. A sound translating device comprising a substantially non-magnetic and electrically conductive frame having a pair of openings therein, a magnetic field structure mounted on said frame and comprising pole pieces having extensions projecting through said openings and a permanent magnet secured between said extensions on one side of said frame, a diaphragm positioned on the other side of said frame, said pole pieces and said permanent magnet being arranged to cooperate with said diaphragm to provide a substantially closed magnetic circuit which is inductively coupled to said frame, and means comprising said frame and a relatively low-reluctance auxiliary flux path extending between said pole pieces on said other side of said frame for preventing at least a portion of the alternating component of flux which is developed during operation of said device from linking with said frame.

6. A sound translating device comprising a substantially non-magnetic andelectrically conductive frame having a pair of openings therein, a magnetic field structure mounted on said frame and comprising pole pieces having extensions projecting through said openings and a permanent magnet secured between said extensions on one side of said frame, a diaphragm positioned on the other side of said frame, said pole pieces and said permanent magnet being arranged to cooperate with said diaphragm to provide a substantially closed magnetic circuit which is inductively coupled to said frame, said pole pieces having projections extending toward each other and separated by an air gap to provide an auxiliary fiux path between said pole pieces on said other side of said frame, and means comprising said frame and said auxiliary flux path for pre-- venting at least a portion of the alternating component of fiux which is developed during operation of said device from linking with said frame.

'7. A sound translating device comprising a substantially non-magnetic and electrically conductive frame having a pair of openings therein, a

magnetic field structure mounted on said frame and comprising pole pieces having extensions projecting through said openings and a permanent magnet secured between said extensions on one side of said frame, a diaphragm positioned on the other side of said frame, said pole pieces and said permanent magnet being arranged to cooperate with said diaphragm to provide a substantially closed magnetic circuit which is inductively coupled to said frame, said pole pieces having projections extending toward each other and separated by an air gap to provide an auxiliary flux path between said pole pieces on said other side of said frame, the length of said air gap being so proportioned that a substantial amount of the steady state fiux traversing said magnetic circuit and produced by said magnet traverses said diaphragm, and means comprising said frame and said auxiliary flux path for shunting a portion of the alternating component of flux which is developed during operation of said device away from said pole piece extensions and said magnet.

8. A sound translating device comprising a substantially non-magnetic and electrically conductive housing having a pair of openings therein, a magnetic field structure mounted on said housing and comprising pole pieces having extensions projecting through said openings and inductively coupled to said housing, a permanent magnet mounted between saidpole piece extensions on the outside of said housing, and means comprising said housing for preventing at least a portion of the alternating component of fiux which is developed during operation of said device from linking with said permanent magnet.

9. A sound translating device comprising a substantially non-magnetic and electrically conductive housing having a pairof openings therein, a magnetic field structure mounted on said housing and comprising pole pieces having extensions projecting through said openings and inductively coupled to said housing, said pole pieces having projections extending toward each other and separated by gap to provide an auxiliary flux path between said pole pieces within said housing, a permanent magnet mounted between said pole piece extensions on the outside of said housing, and means comprising said housing and said auxiliary flux path for preventing at least a portion of the alternating component of flux which is developed during operation of said device from linking with said permanent magnet.

10. A sound translating device comprising a substantially non-magnetic and electrically conductive housing having a pair of openings therein, a magnetic field structure comprising pole pieces having extensions projecting through said openings and inductively coupled to said housing, said pole pieces having projections extending toward each other and separated by a gap to provide an auxiliary flux path between said pole pieces within said housing, means securing said pole piece projections to said housing to provide a rigid assembly, a permanent magnet mounted between said pole piece extensions on the outside of said housing, and means comprising said housing and said auxiliary fiux path for preventing at least a portion of the alternating component of fiux which is developed during operation of said device from linking with said permanent magnet.

11. A sound translating device comprising a substantially non-magnetic housing having a pair of openings therein, a magnetic field structure mounted on said housing and comprising pole pieces having extensions projecting through said openings, a permanent magnet having beveled corners, and means comprising portions of said pole piece extensions bent over to engage the beveled corners of said permanent magnet for rigidly mounting said permanent magnet between said pole piece extensions on the outside of said housing.

12. A sound translating device comprising a substantially non-magnetic housing having a pair of openings therein, a magnetic field structure mounted on said housing and comprising pole pieces having extensions projecting through said openings, a permanent magnet having beveled corners, means comprising portions of said pole piece extensions bent over to engage the beveled corners of said permanent magnet for rigidly mounting said permanent magnet between said pole piece extensions on the outside of said housing, and a substantially U-shaped contact member device of the form comprising a magnetic field structure including a permanent magnet member and pole pieces at least one of which is provided with a winding, a housing for said winding and said pole pieces, and a diaphragm structure; the method of making said device which comprises the steps of arranging said field structure on said housing with said magnet member disposed exteriorly of said housing and with said winding and said pole pieces disposed interiorly of said housing, arranging said diaphragm structure on said housing to provide a substantially sealed chamber enclosing said winding and said pole pieces, all before said magnet member is magnetized, thereby to prevent magnetic particles from being attracted to said field structure during the foregoing steps, and then introducing said magnet member in a magnetic field, thereby to magnetize said magnet member.

15. In the construction of a sound translating device of the form comprising a magnetic field structure including pole pieces having windings thereon and provided with extensions adapted to receive a permanent magnet member therebetween, a cup-shaped housing provided with openings for receiving said pole piece extensions, and a diaphragm structure; the method of making said device which comprises the steps of inserting said pole piece extensions through said openings so that they project exteriorly of said housing, positioning said magnet member exteriorly of said housing and between said pole piece extensions, securing said magnet member between said pole piece extensions, securing said pole pieces to said housing, arranging said diaphragm structure on said housing to provide a substantially sealed chamber enclosing said windings and said pole pieces, all before said magnet member is magnetized, thereby to prevent magnetic particles from being attracted to said field structure during the foregoing steps, and then introducing said magnet member in a magnetic field, thereby to magnetize said magnet member.

HAROLD C. PYE. 

